fix(player): stop PiP dropping the video and restarting the audio behind it

Watching in a picture-in-picture window would occasionally drop to audio-only,
and the audio would resume from wherever the video had been when PiP was
entered while the picture had carried on past it. Two independent faults, both
needed to produce that.

The position froze (DR-265). VideoPlayer tracks the absolute position in its
own `currentTime` rather than reading `videoElement.currentTime` at the point
of use, because transcoded HLS resets the element to 0 on every segment
rebuild. While playing, that variable had exactly one writer: a
requestAnimationFrame loop. RAF is driven by the document being rendered, and
an Android activity behind a PiP window is paused, so the loop stops while the
element plays on. The `timeupdate` handler that would have covered the gap was
written as a fallback "for when RAF isn't running" and gated itself on
`!isPlaying` -- switching itself off at precisely the moment it was the only
source left. Everything downstream froze with it: the seek bar, the ten-second
progress reports, the position mirrored into Rust, and the handoff. The gate is
now `shouldApplyTimeUpdate` and turns only on things that genuinely own the
position -- an in-flight seek, a seek-bar drag, an element below
HAVE_CURRENT_DATA. Both writers producing the same derived value costs nothing.

The handoff fired at all (DR-266). PiP and the background-audio handoff are
alternatives -- one keeps the picture, the other throws it away -- but
exclusivity was enforced from one side only: arming the toggle suppressed
*auto*-PiP, while the PiP button stayed ungated, so pressing it left both
armed. What then stood between them was `isInPictureInPictureMode`, sampled
once inside MainActivity.onStop(). That sample is not reliable: the keyguard
dismissing the window, the window being stashed, or OEM variance in when
onPictureInPictureModeChanged(false) lands can all leave the activity stopped
with a window still on screen and the flag reading false. Now entering PiP
disarms background audio, both directions go through one BackgroundBehaviour
pair, and the PiP question accepts either witness -- the native sample or the
frontend's latch over jellytau-pip-entered/exited. The latch cannot report a
window that has closed: both events reach the WebView through the same message
queue in dispatch order. The decision itself stays in Rust; the frontend only
supplies a fact it can establish more reliably than the activity can.

Red first, both: the existing behaviour was extracted into pure helpers, the
tests written against the correct behaviour, and both watched to fail before
either was changed.

src/lib/components/player/timeTracking.ts

@@ -0,0 +1,45 @@
+/**
+ * Pure helpers for keeping the player's position variable honest.
+ *
+ * TRACES: UR-004, UR-041 | DR-265 | UT-245
+ *
+ * `VideoPlayer.svelte` tracks the absolute playback position in its own
+ * `currentTime` variable rather than reading `videoElement.currentTime` at the
+ * point of use — transcoded HLS resets the element to 0 on every segment
+ * rebuild, so only the component's running total is meaningful. Everything
+ * downstream reads that variable: the seek bar, the progress reports, the
+ * position mirrored into Rust, and the background-audio handoff.
+ *
+ * Which makes "who is allowed to write it" a correctness question, not a
+ * rendering detail — hence a pure module with tests rather than a condition
+ * buried in an event handler.
+ */
+
+export interface TimeUpdateGate {
+  /**
+   * Deliberately does NOT gate the update, and is accepted only to say so.
+   *
+   * `timeupdate` was written as a fallback "for when RAF isn't running" and so
+   * excluded itself whenever `isPlaying` was true. But RAF is driven by the
+   * document being rendered, and an Android activity behind a picture-in-picture
+   * window is paused: the loop stops while the element plays on, and the one
+   * remaining position source had switched itself off. Both writing the same
+   * derived value costs nothing — the element is the authority either way.
+   */
+  isPlaying?: boolean;
+  isSeeking: boolean;
+  isDraggingSeekBar: boolean;
+  readyState: number;
+}
+
+/**
+ * Whether a `timeupdate` event may write the component's position.
+ *
+ * Kept free of Svelte/DOM so the rule is unit-testable without mounting the
+ * player.
+ */
+export function shouldApplyTimeUpdate(opts: TimeUpdateGate): boolean {
+  // An in-flight seek or a drag owns the position until it settles, and an
+  // element with no current data reads 0, which would rewind it.
+  return !opts.isSeeking && !opts.isDraggingSeekBar && opts.readyState >= 2;
+}